A normal eye can see the distant objects as well as the nearby objt clearly.We will now discuss how the eye is able to focus the objects lying at various distances. An eye 👀 focus the image of the distance objects as well as the nearby objects on its retina by changing the focal length (or converging power) of its lens. The focal length of the length eye- lens is changed by the action of ciliary muscles. The ciliary muscles can change the thickness of the soft and flexible eye- lens and hence it's focal length which, in turn, changes the converging power of the eye- lens). Let us see how it happens.
When the eye is looking at a distant object (at infinity), then the ciliary muscles of the eye are fully relaxed. The relaxed ciliary muscles of the eye pull the suspensory ligaments attached to the eye- lens tightly.
The suspensory ligaments, in turn pull the eye- lens due to which the eye- lens gets stretched and becomes thin (or less convex). The thin eye- lens has large focal length but it's converging power is small. The small converging power of thin eye- lens is sufficient to converge the parallel rays of light coming from a distant object to form an image on the retina of the eye 👁️.
When the eye is looking at a distant object, the eye is said to be unaccommoded because it is the relaxed state of the eye. The thin eye- lens is not powerful enough to converge the diverging light rays coming from the nearby objects onto the retina.
So, to look at the nearby objects, the eye- lens has to change its shape and become thick (or more comvex) to increase its converging power. This happens as follows:
When the same eye has to look at a nearby object, the ciliary muscles of the eyes contact. The contracted ciliary muscles make the suspensory ligaments loose. When the suspensory ligaments became loose, they stop pulling the eye- lens. The eye- lens bulges under its own elasticity and becomes thick (or more convex) converging power of eye- lens increase, the thick eye- lens can converge the diverging light rays coming from the nearby object to form an image on the retina of the eye.
This is shown in picture in which an object O is near to the eye. It has been focused by the retina of the eye. This is shown in picture in which an object O is near to the eye. It has been focused by the thick eye- lens to form an image I on the retina. When the eye- lens becomes more convex to focus the nearby objects, the eye is said to be 'accommodated'. We can now say that: The ability of focus the distant objects as well as the nearby objects on the retina by changing the focal length (or converging power) of its lens is called accommodation.
The maximum "accommodation" of a normal eye is reached when the object is at a distance of about 25 cm from the eye. After this the ciliary muscles cannot make the eye- lens more thick (or more convex). So, an object placed at a distance of less than 25 cm cannot be seen clearly by a normal eye because all the power of accommodation of the eye has already been exhausted. Thus, a normal eye has a power of accommodation which enables objects as far as infinity and as close as 25 cm to be focused on the retina. The power of accommodation of the eye for a person having normal vision (normal eyesight) is about 4 dioptres.
